1. Field of the Invention
This invention relates to the field of alkane conversion to alkenes and more specifically to the field of oxidative conversion of alkanes to alkenes in a two-zone reactor.
2. Background of the Invention
There is currently a significant interest in various types of hydrocarbon processing reactions. One such class of reactions involves the chemical conversion of natural gas, a relatively low value reactant, to higher value products. Natural gas comprises several components, including alkanes. Alkanes are saturated hydrocarbons—e.g., compounds consisting of hydrogen (H) and carbon (C)—whose molecules contain carbon atoms linked together by single bonds. The principal alkane in natural gas is methane; however, significant quantities of longer-chain alkanes such as ethane (CH3CH3), propane (CH3CH2CH3) and butane (CH3CH2CH2CH3) are also present Unlike even longer-chain alkanes, these so-called lower alkanes are gaseous under ambient conditions.
The interest in the chemical conversion of the lower alkanes in natural gas stems from a variety of factors. First, vast reserves of natural gas have been found in remote areas where no local market exists. There is great incentive to exploit these natural gas formations because natural gas is predicted to outlast liquid oil reserves by a significant margin. Unfortunately, though, the transportation costs for the lower alkanes are generally prohibitive, primarily because of the extremely low temperatures needed to liquefy these highly volatile gases for transport. Consequently, there is considerable interest in techniques for converting methane and other gaseous hydrocarbons to higher value products and/or more easily transportable products at the remote site. A second factor driving research into commercial methods for chemical conversion of lower alkanes is their abundant supply at many refineries and the relatively few commercially-viable means of converting them to more valuable products.
Several hydrocarbon processing techniques are currently being investigated for the chemical conversion of lower alkanes. One such technique involves the conversion of methane to higher chain-length alkanes that are liquid or solid at room temperature. This conversion of methane to higher hydrocarbons is typically carried out in two steps. In the first step, methane is converted with an oxidant to produce a mixture of carbon monoxide and hydrogen known as synthesis gas or syngas. In a second step, the syngas is converted to liquid hydrocarbon fuels and solid hydrocarbon waxes using the Fischer-Tropsch synthesis. The high molecular weight waxes thus produced provide an ideal feedstock for hydrocracking, which ultimately yields jet fuel, gasoline, high-decane diesel fuel, or blending stocks for such fuels, particularly superior high decane value diesel fuel.
Another important class of hydrocarbon processing reactions relates to the production of olefins from alkanes. Olefins have traditionally been produced from alkanes by fluid catalytic cracking (FCC) or steam cracking, depending on the size of the alkanes. Heavy olefins are herein defined as containing at least five carbon atoms and are produced by FCC. Light olefins are defined herein as containing two to four carbon atoms and are predominantly produced by steam cracking. Olefins can also be generated from low molecular weight alkanes by dehydrogenation reactions. In a dehydrogenation process, alkanes can be dehydrogenated to produce alkenes.
Alkenes, or olefins, are higher value chemicals than their corresponding alkanes. This is true, in part, because alkenes are important feedstocks for producing various commercially useful materials such as detergents, high-octane gasolines, pharmaceutical products, plastics, synthetic rubbers and viscosity additives. In the commercial production of plastics, elastomers, manmade fibers, adhesives, and surface coatings, a tremendous variety of polymers are used. By far the most important industrial polymers are polymerized olefins, which comprise virtually all commodity plastics. Ethylene, a raw material in the production of polyethylene, is the one of the most abundantly produced chemicals in the United States and cost-effective methods for producing ethylene are of great commercial interest.
Olefins are unsaturated hydrocarbons (compounds containing hydrogen [H] and carbon [C]) whose molecules contain one or more pairs of carbon atoms linked together by a double bond. The olefins are classified in either or both of the following ways: (1) as cyclic or acyclic (aliphatic) olefins, in which the double bond is located between carbon atoms forming part of a cyclic (closed-ring) or an open-chain grouping, respectively, and (2) as monoolefins, diolefins, triolefins, etc., in which the number of double bonds per molecule is, respectively, one two, three, or some other number. Hence, olefins are highly desired for the production of plastics.
Generally, olefin molecules are commonly represented by the chemical formula CH2═CHR, where C is a carbon atom, H is a hydrogen atom, and R is an atom or pendant molecular group of varying composition. The composition and structure of R determines which of the huge array of possible properties will be demonstrated by the polymer. More specifically, acyclic monoolefins have the general formula CnH2n, where n is an integer. Acyclic monoolefins are rare in nature but are formed in large quantities during the cracking of petroleum oils to gasoline. The lower monoolefins, i.e., ethylene, propylene, and butylene, have become the basis for the extensive petrochemicals industry. Most uses of these compounds involve reactions of the double bonds with other chemical agents. Acyclic diolefins, also known as acyclic dialkenes, or acyclic dienes, with the general formula CnH2n-2, contain two double bonds; they undergo reactions similar to the monoolefins. The best-known dienes are butadiene and isoprene, used in the manufacture of synthetic rubber.
Olefins containing two to four carbon atoms per molecule are gaseous at ordinary temperatures and pressure; those containing five or more carbon atoms are usually liquid at ordinary temperatures. Additionally, olefins are only slightly soluble in water.
The FCC process is a catalytic thermal process, while steam cracking is a direct, non-catalytic dehydrogenation process. FCC and steam cracking are known to have drawbacks. For example, both processes are endothermic, meaning that heat energy must be supplied to drive the reaction. In addition, in FCC, coke forms on the surface of the catalyst during the cracking processes, covering active sites and deactivating the catalyst. During regeneration, the coke is burned off the catalyst to restore its activity and to provide heat needed to drive the cracking This cycle is very stressful for the catalyst; temperatures fluctuate between extremes as coke is repeatedly deposited and burned off. Furthermore, the catalyst particles move at high speed through steel reactors and pipes, where wall contacts and interparticle contacts are impossible to avoid. The conversion of alkanes to alkenes in both FCC and steam cracking processes may be via multi reaction steps but overall reaction can be explained as a dehydrogenation reaction. One example of such a dehydrogenation reaction is the conversion of ethane to ethylene (Reaction 1):C2H6+Heat→C2H4+H2  (1)
FCC and steam cracking units are large and expensive because the FCC unit requires a catalyst regenerator and its catalysts use typically precious metals, and because the steam cracking unit requires furnaces to generate heat energy for the conversion of alkane to alkene. Recently, there has been increased interest in oxidative dehydrogenation (ODH) as an alternative to FCC and steam cracking for the production of olefins. In ODH, alkanes are dehydrogenated in the presence of an oxidant such as molecular oxygen, typically in a short contact time reactor containing an ODH catalyst. The net ODH reaction, for example as depicted in [Reaction 2] for the conversion of ethane and oxygen to ethylene and water.C2H6+½ O2→C2H4+H2O+Heat  (2)
Because the exothermic combustion provides most of the heat necessary to drive the endothermic dehydrogenation reaction, ODH is a substantially autothermal process and requires no or very little energy to sustain the reaction. Energy savings over traditional, endothermal processes (FCC and steam cracking) can be significant if the heat produced with ODH is recaptured and recycled. In addition, the capital costs for olefin production via ODH are significantly less than with the traditional processes, because ODH uses simple fixed bed reactor designs and high volume throughput.
Although ODH involves the use of a catalyst, which is referred to herein as an ODH catalyst, and is therefore literally a catalytic dehydrogenation, ODH is distinct from what is normally called “catalytic dehydrogenation” in that the former involves the use of an oxidant and the latter does not.
Oxidative dehydrogenation of hydrocarbons (ODH) with short contact time reactors (SCTR) is an alternative to traditional steam cracking and non-oxidative dehydrogenation processes. During an ODH reaction, an oxidant, preferably molecular oxygen, is co-fed with saturated hydrocarbons, optionally balanced with an inert gas, at a gas hourly space velocity (GHSV) of about 20,000 to 10,000,000 hr−1. The oxidant may be fed as pure molecular oxygen, air, oxygen-enriched air, oxygen mixed with a diluent, and so forth. Oxidant in the desired amount may be added in the feed to the dehydrogenation zone. The contact time of the reactants with the catalyst is typically in the 1 to 200 ms range. The reaction pressure range is typically between 0.8 bar and 5 bars (about 80 kPa–500 kPa), and the reaction temperature is typically between 800 and 1,100° C.
Successful commercialization of an ODH process depends on the efficacy of the catalyst. In other words, successful commercial scale operation for catalytic hydrocarbon processing depends upon high hydrocarbon feedstock conversion at high throughput and with acceptable selectivity for the desired reaction products. In turn, the yield and selectivity of an ODH catalyst system are affected by several factors. One of the most important of these factors is the catalyst composition, which significantly affects not only the yields and product distributions but also the overall economics of the process. Unfortunately, few catalysts offer both the performance and cost necessary for economical large-scale industrial use.
Catalyst cost is one of the most significant economic considerations in ODH processes. Non-oxidative dehydrogenation reactions frequently employ relatively inexpensive iron-oxide based catalysts. In contrast, ODH catalysts typically utilize relatively expensive precious metals, e.g., platinum, as promoters that assist in the combustion reaction. In order to reduce catalyst costs, therefore, it is desirable to maximize the effectiveness of the catalyst composition, or minimizing its use.
Consequently, there is a need for an alkane-to-alkene conversion process with an improved conversion and selectivity. Further needs include an improved reactor system for the oxidative conversion of alkanes to alkenes. Additional needs include an improved reactor and process for the management of heat produced in the alkene production process. In addition, needs include a reduction in hydrogen co-product in the alkene production process by the oxidative conversion of alkanes.